Field of Invention
The present invention relates to a micro-electro-mechanical system (MEMS) device, in particular a MEMS device wherein a self-test electrode is accommodated in a self-test frame to form a self-test unit outside a proof mass body.
Description of Related Art
In order to ensure correct operation of a MEMS device, a self-test unit disposed in the MEMS device is a common option. For reference, the followings are related prior art: U.S. Pat. Nos. 7,322,242, 5,983,721, and US Publication 2012/0000287 and US Publication 2013/0214367.
In prior art, there are two typical arrangements to dispose the self-test unit in the MEMS device. In the first arrangement, the self-test unit is a stand-alone unit which is disposed outside the sensing structure of the MEMS device. In the second arrangement, the self-test unit is disposed inside the proof mass body of the MEMS device. U.S. Pat. No. 7,322,242 is an example of the second arrangement.
The first arrangement will significantly increase the overall size of the MEMS device. The second arrangement, although requires less area than the first arrangement, still causes a negative impact on the performance of the MEMS device. That is, because a space needs to be provided inside the proof mass body, the mass quantity of the proof mass body is reduced, and the sensitivity of the MEMS device is reduced. Or, to maintain the same sensitivity, the area occupied by the proof mass body (its mass plus its inner space) needs to be increased.
Further, the prior art second arrangement is only capable of detecting an in-plane offset but can not detect an out-of-plane offset. The term “in-plane” refers to the plane where the proof mass body resides, and the term “out-of-plane” refers to a normal direction perpendicular to the plane where the proof mass body resides.
The present invention provides a solution to the above problem.